Background boosters for elementary teachers
Q: If Energy Is Neither Created Nor
Destroyed, What Happens to It?
By Bill Robertson
BRIAN DISKIN
A:
The one thing we all hear
about energy is that it
is never created nor destroyed. We accept the statement
because, well, we’ve all learned it.
But what exactly happens to energy
in systems so that we can’t just reuse
it? If we can’t destroy energy, then
why isn’t it just hanging around for
us to gather up and put to a good
purpose? The answers to these questions are important for everything
from generating electricity to the
operation of ecosystems.
Grab a book or other solid object
and slide it across a table or a floor.
While the object is sliding, we can
see that it has energy because it’s
moving. But then the object stops,
correct? What happened to the energy of the object? To get a clue as
to what happened to the energy of
the object, rub your hands together.
You’ll notice they get warm. The
motion energy of your hands transfers to the hands themselves (they
get warm) and to the air around
your hands. Similarly, when the object slides across a surface, its energy
gets transferred to the surface and
the object (they both heat up just a
bit) and also to the air surrounding
the surface and the object. Take a
look at Figure 1, p. 76.
Now here’s a challenge for you.
After you’ve slid the object across
the surface, gather up all that energy
that was transferred to the object,
“It’s not laziness, it’s Conservation of Energy.”
the surface, and the surrounding air,
and use it to make the object start
moving again. I’ll wait.
Not happening, right? This situation illustrates what happens in any
energy transfer. The energy gradually becomes less and less “usable.”
We call this the degradation of energy. The energy isn’t lost in the sense
that it disappears, but it is lost in the
sense that it would be extremely difficult, if not impossible, to gather
that energy back up and put it to use.
Here’s another example—the generation of electricity. We generate
electricity in many ways, but most
of the methods involve using some-
thing (water or steam, for example)
to rotate a turbine. The rotating turbine contains magnets and wires,
and it generates electricity. In the
process of generating that electricity, though, a lot of energy is “lost.”
There’s friction in the rotating parts,
which generates heat and transfers a
bunch of energy to the surrounding
air. If you live near a power plant,
you no doubt have seen clouds of
steam rising out of the vents in the
plant. That steam is carrying away
energy that doesn’t contribute to the
generation of electricity.
So, let’s apply what we’ve learned
to what happens in ecosystems. It
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might seem an odd transition, but
energy considerations permeate all
areas of science. At some point in
your learning or teaching, you probably encountered what’s known as
ecological pyramids or energy pyramids. Those are diagrams that show
the relative numbers of primary
producers, such as plants; primary
consumers, such as animals that eat
plants; secondary consumers, such
as animals that eat the animals that
eat plants; and tertiary consumers,
such as animals that eat the animals
that eat ... well, you get the picture.
In case you don’t get the picture,
there’s a drawing in Figure 2.
What might not be obvious in
Figure 2 is why, exactly, it’s a pyramid. Why are there lots and lots of
primary producers, fewer primary
consumers, and fewer and fewer animals on up to the top? Let’s just consider a hawk that consumes a mouse.
The hawk, by using its digestive system and various bodily processes,
obtains energy from the mouse. But
the hawk doesn’t obtain all the energy the mouse expended in its lifetime—the energy the mouse used to
get energy from grains, the energy
the mouse expended in motion, and
the energy the mouse radiated away
as heat throughout its lifetime. In
other words, a lot of the mouse’s
energy is “lost” to the hawk—another example of the degradation
of energy. Similarly, all organisms
at a higher level only benefit from
a small fraction of the total energy
from organisms at a lower level. All
of this “lost” energy means that it
takes a lot of organisms at a lower
level to support a few organisms at a
higher level. Hence, we get the pyramid. This is actually a good thing. If
the energy at each level transferred
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Science and Children
FIGURE 1.
The motion energy of sliding the book transfers to the surrounding
air, the surface, and the book itself. All of them heat up.
FIGURE 2.
Ecological Pyramid
Tertiary consumers
Secondary consumers
Primary consumers
Producers
completely to organisms at the next
level up, we’d have a whole lot more
mice than we do and about as many
hawks as we have mice. It would get
crowded!
Okay, here’s another challenge
for you. You want to light a lightbulb, so you need an energy source.
Head to your nearest pool or lake
or ocean. There’s a whole bunch of
energy there because all of the water
molecules are moving around randomly. If we’re talking about, say,
50 cubic meters of ocean, that’s a
great deal of energy! So, use all that
energy to light your lightbulb. Putting a lightbulb in the ocean doesn’t
make it light, so we’d have to figure
out a different way. We could gather
up that 50 cubic meters of ocean,
make it flow through a turbine, and
use the resulting electricity to light
the bulb. But of course that would
require a great deal of energy input
from us, much more energy than
we’d get in the form of electricity. The lesson here is that you can
use random energy such as that
found in the ocean or that found in
the motion of air molecules around
you, but it requires a large input of
extra energy to do so. And in fact,
we’d “lose” a whole bunch of energy while doing that. The energy of
people’s muscles, the heat radiated
away from people’s bodies, and any
friction involved in our process, results in even more of that “lost” or
degraded energy.
The bottom line is that we can’t
win. Even when we try to gather up
degraded energy, we end up creating more degraded energy. There’s
basically one direction for the flow
of energy in the universe—toward
less and less useful forms (or more
degraded forms) of energy. Another
way of saying that is that energy in
the universe goes from organized to
disorganized.
So maybe you’re wondering what
this means for the ultimate fate of
the universe. There are speculations
that the universe will eventually
reach heat death, in which all of the
energy in the universe becomes so
degraded that it will be impossible
to sustain life or any other organized
process. Fun to think about, but it’s
not as simple as that. Whether or not
we end in heat death has to do with
the expansion rate of the universe,
the role of gravity, and whether or
not we’re correct on that big bang
thing. And even if we were to end
in a heat death, it wouldn’t happen
for billions of years. You have time
to plan. n
Bill Robertson (wrobert9@ix.net
com.com) is the author of the
NSTA Press book series, Stop Faking It! Finally Understanding Science So You Can Teach It.
Visit the NSTA Science
Store at
www.nsta.org/store
to see all of Bill
Robertson’s books
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